Mineralogy - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 67

2.21 Melanite

In leucite phonolites at Tavalato (Italy), melanite occurs both as a phenocryst as
well as groundmass mineral. These phenocrysts are zoned with colourless cores
having pale

brown margins. The analyses show that they are poor in
MgO, and that there is not only a signi

- yellowish -

cant increase in the TiO 2 and Fe 2 O 3
contents but also there is a corresponding decrease in the Al 2 O 3 content in the rims
of the phenocrysts and groundmass phases. Melanite garnets (Table 2.18 ) have also
been described from a melilitite lava

flow at Mt. Vulture (Melluso et al. 1996).

2.22 Carbonate-Bearing Phases

Ankerite phenocysts in association with phlogopite occur in the lamproitic rocks
from the Raniganj Basin, India (Table 2.18 , analyses no. 5). The other accessory
minerals include apatite and chromian spinal in a groundmass of the same phases
along with rutile, pyrite and devitri

ed glass (Gupta et al. 1983). These lamproites
are so much enriched in carbonates (10 60

vol%), that they termed these rocks as carbonated apatite-bearing glimmerites. Basu
et al. (1997) studied the lamproitic rocks of Bokaro basin (Damodar Valley, India).
They also noted the presence of MgCO 3 -rich carbonate phases in these lamproites.

15 vol%, ankarite), and phlogopite (55

- -